The present invention relates to a power steering system or apparatus, and more specifically to a motor-driven hydraulic power steering system.
At very low temperatures at which the viscosity of an operating oil is high, a motor-driven hydraulic power steering system drives an electric motor and circulates the operating oil for warm-up. For the control of the motor, there is provided, in general, a temperature sensor for sensing the temperature of the operating oil. A U.S. Pat. No. 6,161,069 (Fujita)(≈JP 2000-142434 A≈EP 0 999 114 A2) shows power steering apparatus for estimating an oil temperature, without using a temperature sensor, from the temperature of a motor driver element and a motor current.